Thus we interpret the shadow cast by a theory of which either of several pioneers who are diligently climbing may at any time obtain a view. The conductivity of protoplasm (and what is true of its conductivity will be found to hold good equally for its irritability and changeableness of form) is due to the readiness with which its molecules enter into unstable associations with electrolytes. The instability of these associations is related to the tendency of the molecules to cluster. An impulse is passed along a nerve as a displacement of ions; the ions being transferred from one molecule, or group of molecules, to the next. Such an explanation of an impulse involves no chemical breakdown of nerve-substance during its passage along a nerve. It transfers the metabolism which liberates energy (reinforcing the impulses which have originated in sense-organs) to the nerve-cell bodies. It is based upon certain experimental data which appear to have been established; but, like all other hypotheses which are intended to account for physiological phenomena, this one must be brought to the test by varying the conditions under which impulses pass along nerves, and ascertaining whether the consequent alteration in the force, rate, and other attributes of the phenomena are in accordance with physical laws. In applying these tests to the activities of protoplasm, we are, however, met by an insuperable difficulty. The matter which transmits nerve-impulses is alive. We have no laboratory standards by which to judge whether the changes in conduction which are produced by changes in the conditions of the conductor are, or are not, consonant with physical theory. It is with protoplasm that we are dealing, and not with a mixture of proteins in solution. If we surround a nerve with nitrogen, it loses its conductivity in five hours, to recover it when oxygen replaces the neutral gas. This has been regarded as proving that metabolism of the nerve is necessary for the transmission of impulses. But conductivity is a phenomenon of life. Deprivation of oxygen for five hours must bring the nerve-substance to the verge of death. It might be argued that the retention by the nerve for so long a time of its power of conducting impulses shows that its metabolism is not a cause of the phenomenon. Again, it has been shown that warming the nerves of cold-blooded animals greatly increases the rapidity of conduction. It is more than doubled in the nerves of the “foot” of a slug-and a similar increase has been proved for the nerves of a frog-by a rise of temperature of 10° C. Reflecting on the results of this experiment, a physicist would exclaim: “Then an impulse is a wave of chemical change. A rise of 10° C. increases the rate of chemical processes from two to three times; whereas no known physical process is accelerated by more than 5 to 15 per cent.” But the physiologist remembers that a rise of temperature of 10° C. increases all the activities of a frog.